Switching to VoIP

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Chapter 13: Network Infrastructure for VoIP

Chicago voice: 10.1.5.0/255.255.255.0

Chicago future growth: 10.1.6.0/255.254.0.0

Cleveland: 10.1.8.0/255.252.0.0

Cleveland data: 10.1.8.0/255.255.255.0

Cleveland voice: 10.1.9.0/255.255.255.0

Cleveland future growth: 10.1.10.0/255.254.0.0

This way, instead of each location using up 65,000 addresses, they’ll use only 512. In

fact, this example allows growth of 512 more addresses in the future.

Even if you aren’t tunneling traffic—differentiating the mode of ser-

vice by using different subnets for voice and data is still a good way to

organize your network.

Class-of-service prequalification.

In the Cisco configuration sample, you can see the cos

pre-qualify

command. This tells the router to recognize CoS tags within packets

inside the tunnel and then tag the encapsulating packets appropriately. This is cru-

cial for VoIP performance. Without prequalification, the layer 2 and 3 class of ser-

vice tags normally carried by each packet would be encrypted into the tunnel, no

longer legible to routers that are handling the tunnel. The effect would be that those

routers, which cannot see “inside the tunnel,” would think these packets have a reg-

ular priority class of service like any other traffic. Prequalification ensures that the

tunnel packets retain ...